1. Field of the Invention
The invention relates to a non-magnetizable steel casting alloy, the use thereof and a process for manufacturing the alloy.
2. Description of the Prior Art
Heretofore, for cast components for fixing magnet coils and also for highly stressed parts of electrical machines (e.g. single-phase rotary current generators) which may not cause any problems or constrictions of the magnetic flux, preferably austenitic spheroidal cast iron of the type GGG-NiMn 13 7 or GGG-NiMn 23 4 or austenitic cast steel according to basic pig iron specification 390 or ASTM A-296 CF 20 are used.
These alloys all have the disadvantage that the total set of properties required of such parts can not be met by any of these cast iron-carbon alloys, namely,
deep permeability .mu.
homogeneous strength and toughness values up to large wall thicknesses,
structural stability at low temperatures to -196.degree. C. and during changes in temperature,
homogenous magnetic permeability in large solidification cross sections in the range of 100-500 mm even in the residual solidification zone,
machinability and weldability which is at least as satisfactory as for the standard rustproof steel casting alloys, e.g. Material No. 4308 or 4408 (DIN 17 445),
yield point or proof limit of at least 250 N/mm.sup.2,
weldable without micro-cracks.
Depending on the composition, high-alloy CrNiMn cast steel may be fully austenitic or, with an appropriate increase of the Cr content or reduction of the Ni and/or Mn content, may additionally contain more or less high portions of ferrite in the austenitic basic structure. The austenitic phase is non-magnetic, with a very low magnetic permeability (.mu..ltoreq.1.001), while the ferritic phase is ferromagnetic with correspondingly high permeability values. For this reason, in two-phase austenitic-ferritic alloy, the magnetic permeability strongly increases together with the ferrite content (FIG. 5). Therefore, for so-called non-magnetic alloys with very low permeability, exclusively fully austenitic alloys are used if these steels are to be used as wrought alloys. This course can not be taken in the case of cast steel, since the alloys cannot subsequently deformed. Due to their high susceptibility to heat cracks during welding, fully austenitic CrNiMo steel casting alloys are practically not weldable without cracking. This problem does not arise in the wrought alloys (forging steels and rolled steels) to such an extent since these steels are much stabler in respect to the susceptibility to heat cracks during welding as a result of the deformation and the consequently possible subsequent change in granulation of the structure through recrystallization by means of a heat treatment.
As is well known, the weldability of the cast CrNiMn steel alloys is improved significantly, when these alloys have certain ferrite contents. In this regard, it is not important how much ferrite these alloys have in the state of use, for example, at room temperature, but what ferrite contents they have during the welding state. During welding, i.e. in the state of equilibrium in the vicinity of the melting point, the ferrite content should be about 5%. Our tests have shown that these ferrite contents are already achieved during the welding of alloys when they contain only about 2% ferrite in the cast state at room temperature.
It is true that CrNiMn steel casting alloys with ferrite contents of more than 2% are known; however, they can not be used as non-magnetic, rustproof steel castings since the permeability is too high due to the ferrite content.